Widely and rapidly tunable lasers are important for a variety of detection, communication, measurement, therapeutic, sample modification, and imaging systems. For example, swept source optical coherence tomography (SSOCT) systems employ repetitively swept tunable lasers to generate subsurface microstructural images of a wide range of materials. In SS-OCT, wide tuning range translates to higher axial measurement resolution, and higher tuning speed enables real-time acquisition of large data sets. In addition, variable tuning speed enables trading off imaging range and resolution as required for different applications. Lastly, long coherence length, which is equivalent to narrow linewidth, enables long imaging range. Another example of a system which requires rapidly and widely tunable lasers is transient gas spectroscopy as, for example, described in (Stein, B. A., Jayaraman, V. Jiang, J. J, et al., “Doppler-limited H20 and HF absorption spectroscopy by sweeping the 1321-1354 nm range at 55 kHz repetition rate using a single-mode MEMS-tunable VCSEL,” Applied Physics B: Lasers and Optics 108(4), 721-5 (2012)). In gas spectroscopy, tuning speed enables characterization of time-varying processes, such as in engine thermometry. Narrow spectral width enables resolution of narrow absorption features, such as those that occur at low gas temperatures. Other transient spectroscopic applications include monitoring of explosive or other non-repetitive processes.
Beyond wide tunability and long coherence length, other important parameters for tunable lasers for a variety of applications include tuning speed and variability of tuning speed. In SS-OCT, increased tuning speed enables imaging of time-varying physiological processes, as well as real-time volumetric imaging of larger data sets. Also for SS-OCT, variability of tuning speed enables switching between high speed, high resolution short-range imaging, and low speed, low resolution long range imaging in a single device, which is of great utility in, for example, ophthalmic imaging, as described in (Grulkowski, I., Liu, J. J., Potsaid, B. et al., “Retinal, anterior segment and full eye imaging using ultrahigh speed swept source OCTwith vertical-cavity surface emitting lasers,” Biomed. Opt. Express, 3(11), 2733-2751 (2012)). Spectroscopic or other detection applications benefit in analogous ways from high-speed and variable speed.
Further desirable properties of widely tunable lasers include high output power, center wavelength flexibility, spectrally shaped output, monolithic and low-cost fabrication, and compatibility with array technology. High power increases signal to noise ratio for virtually every application. Center wavelength flexibility translates into greater utility in a larger variety of applications. Spectrally shaped output also increases signal to noise ratio and improves thermal management. Monolithic, low cost fabrication has obvious advantages, and array technology simplifies applications in which multiple sources are multiplexed.
The limitations of prior art tunable lasers with respect to the desirable properties above can be understood by examination of three representative examples. These examples include Fourier Domain mode-locked (FDML) lasers, external cavity tunable lasers (ECTL), and sampled grating distributed bragg reflector (SGDBR) lasers. An FDML laser is described in (Huber, R., Adler, D. C., and Fujimoto, J. G., “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s,” Optics Letters, 31(20), 2975-2977 (2006)). Use of a commercial ECTL in an SSOCT system is described in (George, B., Potsaid, B., Baumann, B., Huang, D. et al., “Ultrahigh speed 1050 nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second,” Optics Express, 18(19), 20029-20048 (2010)). Operation of an SGDBR laser is described in (Derickson, D., “High-Speed Concatenation of Frequency Ramps Using Sampled Grating Distributed Bragg Reflector Laser Diode Sources for OCT Resolution Enhancement,” Proceedings of the SPIE—The International Society for Optical Engineering, 7554, (2010)). FDML and ECTL devices are essentially multi-longitudinal mode devices, which sweep a cluster of modes instead of a single mode across a tuning range. This results in limited imaging range for SSOCT and limited spectral resolution for spectroscopic applications. Both FDML and ECTL are also non-monolithic sources, which are assembled from discrete components, and therefore not low cost devices or compatible with array fabrication. The ECTL further suffers from fundamental speed limitations of about 100 kHz repetition rate or less, due to the long time delay in the external cavity, as described in (Huber, R., Wojtkowski, M., Taira, K. et al., “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Optics Express, 13(9), 3513-3528 (2005).) Further speed limitations in ECTL devices arise from the large mass of the grating tuning element, as for example in the commercially available Thorlabs model SL1325-P16 grating tuned laser. The FDML suffers also from inflexiblity of both center wavelength and tuning speed. Since the FDML employs a long fiber-based cavity, it can only operate at wavelengths where low-loss optical fiber is readily available. Secondly, the FDML sweep rate is fixed by the roundtrip time of light in the fiber external cavity, and variable sweep rates are therefore not possible in a single devices.
The SGDBR is a single transverse and longitudinal mode device, and has the potential for long imaging range and narrow spectral width. Tuning, however, is accomplished by discontinuous hopping amongst various modes, which tends to introduce measurement artifacts. The mode-hopping also requires multiple tuning electrodes, complicated drive circuitry and associated speed limitations. The SGDBR also suffers from limited tuning range relative to external cavity and FDML lasers, since the latter use lossless tuning mechanisms, while the SGDBR is tuned by free carrier injection, which introduces free carrier losses and limits tuning range. The SGDBR also suffers from center wavelength inflexibility, due to the need for complex regrowth fabrication technology which is only mature in the Indium Phosphide material system.
The problems discussed above with respect to the FDML, ECTL, and SGDBR above are representative of problems encountered by most tunable lasers known in the art.
MEMS-tunable vertical cavity lasers (MEMS-VCSELs) offer a potential solution to the problems above. The short cavity of MEMS-VCSELs leads to a large longitudinal mode spacing and relative immunity to mode hops. The MEMS-VCSEL requires only one tuning electrode to sweep a single mode across the tuning range, and therefore offers the promise of long SS-OCT imaging range with minimal measurement artifacts, and rapid tuning. The short cavity and the short mass of the MEMS mirror offer the potential for very high speed. MEMS-VCSEL technology can also be extended to a large variety of wavelength ranges difficult to access with many other types of sources, making them appropriate for other types of spectroscopic, diagnostic, and detection systems. The application of MEMS-VCSELs to SS-OCT imaging was first described in U.S. Pat. No. 7,468,997. MEMS-VCSELs have the potential for wide tuning range, as discussed in U.S. Pat. No. 7,468,997. Until 2011, however, the widest MEMS-VCSEL tuning range achieved was 65 nm around 1550 nm, as described in (Matsui, Y., Vakhshoori, D., Peidong, W. et al., “Complete polarization mode control of long-wavelength tunable vertical-cavity surface-emitting lasers over 65-nm tuning, up to 14-mW output power,” IEEE Journal of Quantum Electronics, 39(9), 1037-10481048 (2003). This represents a fractional tuning range of about 4.2%, or about a factor of 2 less than that required in SS-OCT imaging.) In 2011, a tuning range of 111 nm was demonstrated in a 1310 nm MEMS-VCSEL, which was subsequently applied in an SSOCT imaging system, as described in (Jayaraman, V., Jiang, J., Li, H. et al., “OCT Imaging up to 760 kHz Axial Scan Rate Using Single-Mode 1310 nm MEMS-Tunable VCSELs with >100 nm Tuning Range,” CLEO: 2011—Laser Science to Photonic Applications, 2 pp.-2 pp. 2 pp. (2011).)
The MEMS-VCSEL described by Jayaraman, et al. in 2011 represented a major innovation in widely tunable short cavity lasers. Achieving performance and reliability appropriate for commercial optical systems, however, requires optimization of tuning speed, frequency response of tuning, tuning range, spectral shape of tuning curve, output power vs. wavelength, post-amplified performance, gain and mirror designs, and overall cavity design. Numerous design innovations are required to improve upon the prior art to achieve performance and reliability necessary for these commercial systems.
From the foregoing, it is clear that what is required is a widely tunable short-cavity laser with 3-dimensional cavity and material design optimized for performance and reliability in SSOCT imaging systems, spectroscopic detection systems and other types of optical systems.